Over the years, common rail fuel systems for compression ignition engines have been gaining acceptance in the industry. A typical common rail fuel system includes a high pressure pump that is driven directly via linkage by the engine crank shaft to supply high pressure fuel to a common rail. Individual fuel injectors are positioned for direct injection of fuel into individual engine cylinders, and each fuel injector is fluidly connected to the common rail via an individual branch passage. The high pressure pump will typically include from one to six reciprocating pump plungers that are each driven by individual or shared cams that include typically from one to six lobes per cam. As the cam rotates, each lobe causes its associated plunger(s) to reciprocate at least once for an individual camshaft rotation. The number of strokes is dependent upon the detailed shape of the camshaft lobe. The lobe can be shaped to provide 1, 2, 3, or more integer numbers of plunger strokes per lobe per camshaft revolution. Although the cam(s) are driven to rotate directly by the engine crank shaft via a linkage, designers can select a linkage to provide any suitable ratio of engine speed to pump speed. The number of pumping events per engine cycle, which corresponds to 720° of rotation for a four cycle engine, can be calculated by multiplying the ratio of the pump speed to engine speed times two, and multiplying that product by the product of the number of pump plungers times the number of cam lobes times the number of plungers strokes per camshaft rotation provided by the camshaft lobe shape.
Almost all common rail fuel systems utilize an electronic controller with a feedback control system to control pressure in the common rail while the engine is operating. The problem of rail pressure control has plagued engineers for many years since the rail pressure has a tendency to fluctuate due to the fact that fuel is leaving the common rail for fuel injection events on an intermittent basis, and fuel is being supplied to the common rail in a less than steady state fashion corresponding to individual sequential pumping events. In many cases, a rail pressure sensor will provide information to the electronic controller that will then compare that sensed pressure to a desired pressure, and determine an error. This error will typically be multiplied by some gain in order to determine an adjustment to the output rate from the high pressure pump to move the sensed common rail pressure closer to the desired pressure. For instance, the controller may command one or more spill valves associated with the high pressure pump to close at particular timings to change the output rate from the pump by displacing only a fraction of the pump plunger's displacement toward the common rail, with the remaining portion of that pump plunger's displacement being recirculated at low pressure. In other systems, pump output is controlled by throttling inlet flow with an electronically controlled valve. These strategies often utilize intensive numerical processing that may or may not include filtering of pressure sensor measurements in the highly dynamic environment of common rail pressure associated with a nearly incompressible liquid (diesel fuel) while pumping and injection events are intermittently occurring.
An improvement on this basic feedback control strategy is described in co-owned U.S. Pat. No. 6,484,696. This system reduces the time lag in correcting the rail pressure via reliance on a model based strategy for anticipating fuel arriving and leaving the common rail so that the feedback controller need only correct errors between the model and the actual amount of fluid arriving and leaving at the common rail. The end result being tighter control and less time lag in removing errors in rail pressure. While these strategies have proven successful in controlling rail pressure, engineers have come to recognize that holding rail pressure steady in the highly dynamic environment of fuel leaving and arriving at different times to the common rail at different rates is very problematic. Those skilled in the art will appreciate that injection rates are generally proportional to rail pressure at the time the fuel injector nozzle opens. Thus, fluctuating rail pressures will inherently lead to some uncertainty in fuel injection rates and amounts, which can degrade both performance, increase undesirable emissions, and cause undesirable noise and vibrations.
One strategy for supposedly decreasing common rail pressure variations is taught in U.S. Pat. No. 6,763,808. This reference teaches the use of asymmetrical cam lobes to reduce drive torque variations, and hence supposedly reduce both pressure variations in the common rail and potentially lead to lower noise in the linkage that connects the pump drive shaft to the engine crank shaft. Noise in the linkage can occur generally due to the cyclic torques occurring in the linkage due to the cam lobes being loaded and unloaded as each pump plunger undergoes its pumping stroke and then passes through its top dead center position. As the industry demands ever higher injection pressures in order to improve performance and decrease undesirable emissions, noise and vibration issues generated in the linkage connecting the engine crank shaft to the high pressure common rail pump drive shaft can become more problematic. These vibrations can lead to early failure in the linkage. In addition, these problems are compounded by the fact that some jurisdictions are now prescribing noise limits for engines that are becoming increasingly hard to satisfy.
Another problem constantly plaguing engine manufacturers is how to leverage pump design for a proven application into a new engine. For instance, those skilled in the art will recognize that newly designing a pump for every different engine in a family of engines from a single manufacturer can be extremely expensive and time consuming. On the other hand, for utilizing technologically proven pumps with little or no modification in a family of different engines could be very cost effective. However, doing this has proven extremely difficult to accomplish in practice. For instance, the same pump used in a six cylinder engine equipped with a common rail fuel system when used in a four cylinder engine may produce excessive noise and vibrations, along with less than ideal rail pressure stability.
The present disclosure is directed toward one or more of the problems set forth above and/or other problems.